Pointers & dereferencing in C#

frank297 · July 13th, 2004, 01:21 PM

I am wrapping a Dll function with managed code and am confused about the equivalent of pointers and the & symbol in C. I am working with two C functions wrapped in a Dllimport:

[DllImport("msg.dll",CharSet=CharSet.Ansi)]
public static extern int MsgConnect (int lAPIVersion, string lpcszTopic, ref OPERATOR lpOperator, ref IntPtr lphContext);

Where the C functional prorotype was:
MSGAPIFUNCTION MsgConnect(long lMsgAPIVersion, const char *lpcszTopic, LPOPERATOR lpOperator, MSGHANDLE *lphContext);

The lphContext is a handle, specified in C as:
typedef void *MSGHANDLE

I am trying to import this function:
MSGAPIFUNCTION MsgStartMessage(MSGHANDLE hContext, const char *lpcszActivityName, const char *lpcszMsgName, int bOriginatorVerified)

I have defined it as:
[DllImport("msg.dll",CharSet=CharSet.Ansi)]
public static extern int MsgStartMessage (ref IntPtr hContext, ref string lpcszActivityName, ref string lpcszMsgName, int bOriginatorVerified);

Note it is using the variable itself, not a pointer to it like the previous prototype. I implemented the pointer as IntPtr, not sure how to implement this - do I need to convert it to an Int?

Kep · July 13th, 2004, 06:34 PM

Just so I understand you right. The "MSGHANDLE *lphContext" parameter is a) used by the external function and/or b) set by the external function?

I think from what you're saying is that it is set by the function. You connect to the Message and it returns a handle to it if successful.

I've never done this before but I think what you need to do is allocate a piece of memory for the pointer to be stored in. So I suggest removing the 'ref' modifier on the lphContext parameter and just have it as an IntPtr.

Before calling the function you need to create the memory for the new handle.

Code:

IntPtr lphContext = Marshal.AllocHGlobal(IntPtr.Size);
IntPtr actualHandle;

Marshal.WriteIntPtr(lphContext, IntPtr.Zero);
MsgConnect(/*ver*/1, /*topic*/"blah", /*operator*/someOperator, lphContext);
// if successful
actualHandle = Marshal.ReadIntPtr(lphContext);

Basically this creates a block of memory for the handle and POINTS lphContext at that block. You can then read and write from that block using ReadIntPtr and WriteIntPtr.

If I've understood correctly this should solve your problem.

P.S. Don't forget to remove the "ref" from the lphContext parameter declaration.

Kep.